This invention relates to rolled aluminum alloy sheets intended for forming and a method for making the same. More particularly, it relates to rolled aluminum alloy sheets intended for forming and suitable for use in applications requiring a high strength and paint coating by baking prior to use, for example, as automobile bodies, as well as a method for making the same.
In the prior art, most body sheets used in automobile bodies were cold rolled steel sheets. Because of the recent increasing demand for lighter weight bodies, a great attention has been drawn to rolled aluminum alloy as one of substitutes for steel. Since automobile body sheets are press formed and coated with paint by baking prior to use, a number of requirements are imposed on them, including excellent formability or workability, particularly elongation and stretchability, controlled development of Luders' marks during forming process, high strength, and maintenance of strength after paint baking.
There are known in the prior art a variety of aluminum alloy sheets for use as formed parts requiring a certain strength. They are generally classified into the following types in accordance with alloy composition.
(A) 0-tempered (fully annealed) 5052 alloy (Mg 2.2-2.8%, Cr 0.15-0.35%, balance essentially Al) and 0-tempered 5182 alloy (Mn 0.20-0.50%, Mg 4.0-5.0%, balance essentially Al) which are both non-heat-treatable Al-Mg alloys. PA1 (B) T.sub.4 -tempered 2036 alloy (Cu 2.2-3.0%, Mn 0.1-0.4%, Mg 0.3-0.6%, balance essentially Al) which is a heat-treatable Al-Cu alloy. PA1 (C) T.sub.4 -tempered, heat-treatable Al-Mg-Zn-Cu alloys. Exemplary of these aluminum alloys are those alloys disclosed and claimed in Japanese Patent Application Kokai Nos. 52-141409, 53-103914, and 57-98648. PA1 (D) T.sub.4 -tempered 6009 alloy (Mg 0.4-0.8%, Si 0.6-1.0%, Cu 0.15-0.6%, Mn 0.2-0.8%, balance essentially Al) and 6010 alloy (Mg 0.6-1.0%, Si 0.8-1.2%, Cu 0.15-0.6%, Mn 0.2-0.8%, balance essentially Al) which are both heat treatable Al-Mg-Si alloys. PA1 1.2% to 2.5% of silicon (Si), PA1 0.25% to 0.85% of magnesium (Mg), PA1 0.05% to 0.4% of iron (Fe), PA1 0.1% to 1.5% of copper (Cu), and PA1 at least one member selected from the group consisting of 0.05% to 0.6% of manganese (Mn), 0.05% to 0.3% of chromium (Cr), and 0.05% to 0.15% of zirconium (Zr), balance essentially aluminum.
These prior art aluminum alloys, however, fail to fully satisfy all of the aforementioned requirements imposed on automobile body sheets.
More specifically, alloys (A) have insufficient strength and are susceptible to Luders' marks during forming process. The strength of alloys (A) is further reduced by paint baking. Alloys (B) have less formability and tend to lower their strength during paint baking. Alloys (C) are not fully satisfactory in formability, particularly bending properties and also tend to lower their strength during paint baking. Among alloys (D), for example, 6009 alloy has an insufficient strength and 6010 alloy is poor in elongation and bending.
Few of the conventional aluminum alloys can fully satisfy the aforementioned requirements imposed on automobile body sheets, including excellent formability, particularly elongation and stretchability, absence of Luders' marks, and high strength, particularly after paint baking.